Gram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol. Many species of Gram-negative bacteria are pathogenic, meaning that they can cause disease in a host organism. This pathogenic capability is usually associated with certain components of Gram-negative cell walls, in particular the lipopolysaccharide (also known as LPS or endotoxin) layer. LPS is a major component of the outer membrane of Gram-negative bacteria, contributing greatly to the structural integrity of the bacteria, and protecting the membrane from certain kinds of chemical attack. LPS also increases the negative charge of the cell membrane and helps stabilize the overall membrane structure. LPS is an endotoxin, and induces a strong response from normal animal immune systems. LPS is additionally an exogenous pyrogen (external fever-inducing compound). LPS comprises three parts: polysaccharide (O) side chains, a core polysaccharide and lipid A.
Lipid A contains unusual fatty acids (e.g. hydroxy-myristic acid) and is embedded in the outer membrane while the rest of the LPS projects from the surface. Lipid A is a disaccharide with multiple fatty acid tails reaching into the membrane. When bacterial cells are lysed by the immune system, fragments of membrane containing lipid A are released into the circulation, causing fever, diarrhea, and possible fatal endotoxic shock (also called septic shock).
The polysaccharide side chain is referred to as the O-antigen of the bacteria. O side chain (O-antigen) is a polysaccharide chain that extends from the core polysaccharide. The composition of the O side chain varies between different Gram-negative bacterial strains. O side chains are easily recognized by the antibodies of the host, however, the nature of the chain can easily be modified by Gram-negative bacteria to avoid detection.
The core oligosaccharide contains unusual sugars (e.g. KDO, keto-deoxyoctulosonate and heptose), but little is known concerning its role. In particular, its role in virulence has never been studied directly.
Numerous LPS mutants inducing humoral immunity to lipopolysaccharide (LPS) have been proposed as potential vaccines. However, pure LPS mutants or bacteria expressing LPS mutants are generally considered too toxic to be used as vaccines, in particular in view of their strong adverse effects, and there is thus a need for new vaccines, presenting an attenuated virulence and inducing a sufficient humoral immunity for ensuring vaccination of the host.